1. Technical Field
The present invention relates to an active matrix liquid crystal display device and, more particularly, to an active matrix liquid crystal display device that drives liquid crystal molecules by applying voltage between an upper electrode layer and a lower electrode layer that are formed on the same substrate through an insulating layer, wherein one of the upper electrode layer and the lower electrode layer is a common electrode layer and the other is a pixel electrode layer, and a plurality of openings are formed in the upper electrode layer and arranged parallel to each other so that an electric field is passed therethrough.
2. Related Art
In regard to a display type of an existing active matrix liquid crystal display device, a TN (twisted nematic) type display device has been widely used. However, the TN type display device has a limited viewing angle due to its display principle. A horizontal electric field type display device has been known as a solution for the above problem, in which a pixel electrode and a common electrode are formed on the same substrate, voltage is applied between the pixel electrode and the common electrode to thereby generate an electric field that is substantially parallel to the substrate, and liquid crystal molecules are thus driven mainly within a plane parallel to the surface of the substrate.
In the horizontal electric field type display device, an IPS (in plane switching) mode display device and an FFS (fringe field switching) mode display device are known. In the IPS mode display device, a comb-shaped pixel electrode and a comb-shaped common electrode are arranged in combination. In the FFS mode display device, an upper electrode layer and a lower electrode layer are formed via an insulating layer, in which one of the upper electrode layer and the lower electrode layer is used as a common electrode layer and the other is used as a pixel electrode layer, and then openings, such as slits, are formed in the upper electrode layer so that an electric field is passed therethrough.
In regard to the insulating layer interposed between the upper electrode layer and the lower electrode layer, Japanese Unexamined Patent Application Publication No. 2001-183685 describes an example of an insulating layer, which is constituted of a single surface protection insulating layer for TFT or an example of an insulating layer, which is constituted of a gate insulating film for TFT, interposed between a pixel electrode and a common signal electrode, which are made of upper and lower two ITO layers.
In the active matrix liquid crystal display device, a holding capacitor is provided for restricting changes in electric potentials of pixels when liquid crystals are driven. In the case of FFS mode display device, using an insulating layer interposed between an upper electrode layer and a lower electrode layer, a capacitor formed at the portion at which the upper electrode layer overlaps the lower electrode layer may be used as a holding capacitor. However, when the capacitor formed at the portion at which the upper electrode layer overlaps the lower electrode layer is used as the holding capacitor, the capacitance of the holding capacitor varies depending on the pixel density of the liquid crystal display device. For example, as the pixel density is increased and the area of one pixel on the plane is reduced, the capacitance of each holding capacitor is reduced. In contrast, as the pixel density is reduced and the area of one pixel on the plane is increased, the capacitance of each holding capacitor is increased.